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Adding acid treated fish bone meal in the feed increases the salmon’s uptake of phosphorus by a full 14 percent, fresh research by Nofima shows.

Fish and mammals need phosphorus to achieve normal growth, metabolism and skeletal development. Consequently, phosphorus is an important ingredient in feeds for farmed salmon.

Historically, fishmeal has been the main source of phosphorus in the salmon feed. However, in recent times increasingly more of the meal has been replaced by vegetable ingredients, which can in the worst case prevent the fish taking up phosphorus.

Phosphorus in fish bones – poorly available

In order to secure sufficient phosphorus for the salmon, it is common to use commercial phosphorus salts as additives in the feed. The official recommendation in Norway is that there should be minimum 0.6 % available phosphorus in the feed in order for the salmon to grow well and to avoid the development of skeletal deformities. However, research carried out by Nofima demonstrates that the optimal level is higher and about 0.8 – 0.9 % available phosphorus.

Increasing the phosphorus content will make the feed more expensive. Phosphorus is a non-renewable resource, and as phosphorus is in high demand as a fertiliser and there is limited supply of phosphorus from natural sources, prices have been forced up. Consequently, in aquaculture it is necessary to have the best possible utilisation of the phosphorus that is available.

Most of the phosphorus in fishmeal can be found in the fish bones. In the fish bones, phosphorus is tightly bound in a mineral complex with low solubility, making the phosphorus poorly available to the salmon. This means that 60 – 80 % of the phosphorus is released undigested into the environment, which in addition to posing an environmental challenge is a waste of an expensive feed ingredient. Today’s situation increases the need to develop feed ingredients that are more efficient and environmentally friendly.

Improved phosphorous efficiency

Nofima is working on the development of methods to increase the utilisation of phosphorus. Fishmeal, which has historically been the greatest source of phosphorus, is being used in fish feed to a steadily lower degree. Large quantities of by-products are being produced in the form of head and backbone trimmings from the fishery and aquaculture industries. The protein and fat from the trimmings is regarded as a valuable resource, while the bone fraction has been assessed as problem waste.

However, it is in the fish bones that the phosphorus is found. The question scientists at Nofima raised was whether it was possible to grind the fish bones and process the raw material in a way that can make the phosphorus more soluble and available to the salmon? The scientists have now tested this.

“The results have been very positive. If you use the phosphorus from the fish bones, they have higher efficiency and can completely replace other phosphorus salts in the feed,” says Senior Scientist Sissel Albrektsen from Nofima in Bergen.

Positive for growth and skeletal development

The scientists tested out several diets containing phosphorus from different sources, either from commercial salts or from dried fish bone meal that had undergone various acid treatments. The experimental diets were fed to salmon weighing 170 g for a three-month feeding period. Measurements of the fish were then carried out based on a series of criteria.

The feed containing acid treated phosphorus from fish bones increased the salmon’s uptake of phosphorus by 14 %. The ability to digest proteins, fat and several other minerals also increased significantly with this diet.

“We also found effects that we had not counted on. The fish that received this feed had a better growth rate than the other fish groups, and not a single fish had developed deformities,” says Albreksten.

“As fish bones contain many different trace minerals in addition to macrominerals such as phosphorus, calcium and magnesium, we believe that fish bones produced in this way can give a more balanced supplement of minerals that can be favourable for growth and normal skeletal development.”

From blue whiting to herring

The fish bones utilised in this research are from blue whiting. However, increasingly more fishmeal today comes from herring. Albrektsen and her colleagues will now research the same relevant issues on herring bones.

“Herring bone is totally different to blue whiting bone. It is softer and contains more fat and less minerals. We will study if the same processing of the raw material will produce the same effect,” says Albrektsen.

The scientists also plan to conduct a cost-benefit analysis to find out the degree to which it will be profitable to produce feed using phosphorus from fish bones instead of from expensive salts.